This invention relates to interferometers, e.g., displacement measuring and dispersion interferometers that measure displacements of a measurement object such as a mask stage or a wafer stage in a lithography scanner or stepper system, and also interferometers that monitor wavelength and determine intrinsic properties of gases.
Displacement measuring interferometers monitor changes in the position of a measurement object relative to a reference object based on an optical interference signal. The interferometer generates the optical interference signal by overlapping and interfering a measurement beam reflected from the measurement object with a reference beam reflected from the reference object.
In many applications, the measurement and reference beams have orthogonal polarizations and different frequencies. The different frequencies can be produced, for example, by laser Zeeman splitting, by acousto-optical modulation, or internal to the laser using birefringent elements or the like. The orthogonal polarizations allow a polarizing beam splitter to direct the measurement and reference beams to the measurement and reference objects, respectively, and combine the reflected measurement and reference beams to form overlapping exit measurement and reference beams. The overlapping exit beams form an output beam that subsequently passes through a polarizer. The polarizer mixes polarizations of the exit measurement and reference beams to form a mixed beam. Components of the exit measurement and reference beams in the mixed beam interfere with one another so that the intensity of the mixed beam varies with the relative phase of the exit measurement and reference beams. A detector measures the time-dependent intensity of the mixed beam and generates an electrical interference signal proportional to that intensity. Because the measurement and reference beams have different frequencies, the electrical interference signal includes a xe2x80x9cheterodynexe2x80x9d signal having a beat frequency equal to the difference between the frequencies of the exit measurement and reference beams. If the lengths of the measurement and reference paths are changing relative to one another, e.g., by translating a stage that includes the measurement object, the measured beat frequency includes a Doppler shift equal to 2 xcexdnp/xcex, where v is the relative speed of the measurement and reference objects, xcex is the wavelength of the measurement and reference beams, n is the refractive index of the medium through which the light beams travel, e.g., air or vacuum, and p is the number of passes to the reference and measurement objects. Changes in the relative position of the measurement object correspond to changes in the phase of the measured interference signal, with a 2xcfx80 phase change substantially equal to a distance change L of xcex/(np), where L is a round-trip distance change, e.g., the change in distance to and from a stage that includes the measurement object.
Unfortunately, this equality is not always exact. Many interferometers include nonlinearities such as what are known as xe2x80x9ccyclic errors.xe2x80x9d The cyclic errors can be expressed as contributions to the phase and/or the intensity of the measured interference signal and have a sinusoidal dependence on the change in optical path length pnL. In particular, the first order cyclic error in phase has a sinusoidal dependence on (2xcfx80pnL)/xcex and the second order cyclic error in phase has a sinusoidal dependence on 2(2xcfx80pnL)/xcex. Higher order cyclic errors can also be present.
Cyclic errors can be produced by xe2x80x9cbeam mixing,xe2x80x9d in which a portion of an input beam that nominally forms the reference beam propagates along the measurement path and/or a portion of an input beam that nominally forms the measurement beam propagates along the reference path. Such beam mixing can be caused by ellipticity in the polarizations of the input beams and imperfections in the interferometer components, e.g., imperfections in a polarizing beam splitter used to direct orthogonally polarized input beams along respective reference and measurement paths. Because of beam mixing and the resulting cyclic errors, there is not a strictly linear relation between changes in the phase of the measured interference signal and the relative optical path length pnL between the reference and measurement paths. If not compensated, cyclic errors caused by beam mixing can limit the accuracy of distance changes measured by an interferometer. Cyclic errors can also be produced by imperfections in transmissive surfaces that produce undesired multiple reflections within the interferometer and imperfections in components such as retroreflectors and/or phase retardation plates that produce undesired ellipticities in beams in the interferometer. For a general reference on the theoretical cause of cyclic error, see, for example, C. W. Wu and R. D. Deslattes, xe2x80x9cAnalytical modelling of the periodic nonlinearity in heterodyne interferometry,xe2x80x9d Applied Optics, 37, 6696-6700, 1998.
In dispersion measuring applications, optical path length measurements are made at multiple wavelengths, e.g., 532 nm and 1064 nm, and are used to measure dispersion of a gas in the measurement path of the distance measuring interferometer. The dispersion measurement can be used to convert the optical path length measured by a distance measuring interferometer into a physical length. Such a conversion can be important since changes in the measured optical path length can be caused by gas turbulence and/or by a change in the average density of the gas in the measurement arm even though the physical distance to the measurement object is unchanged. In addition to the extrinsic dispersion measurement, the conversion of the optical path length to a physical length requires knowledge of an intrinsic value of the gas. The factor xcex93 is a suitable intrinsic value and is the reciprocal dispersive power of the gas for the wavelengths used in the dispersion interferometry. The factor xcex93 can be measured separately or based on literature values. Cyclic errors in the interferometer also contribute to dispersion measurements and measurements of the factor xcex93. In addition, cyclic errors can degrade interferometric measurements used to measure and/or monitor the wavelength of a beam.
The invention relates to the compensation of cyclic errors in interferometric measurements, such as those used in microlithography systems that fabricate integrated circuits. The interferometric measurements can include changes in a linear displacement of an object, changes in angular orientation of an object, and/or changes in the propagation direction of an optical beam. A number of the embodiments involve heterodyne interferometry and the calculation of a superheterodyne signal corresponding to the product of two signals derived from corresponding intensity measurements of interferometric output beams having orthogonal linear polarizations. The phase of the superheterodyne signal provides information about the different paths traversed by the reference and measurement beam components of the interferometric output beams (e.g., a displacement and/or angle measurement). The calculation of the superheterodyne signal eliminates or substantially reduces first-order cyclic error terms in its phase, thereby improving the accuracy of the information derived from the phase of the superheterodyne signal.
Moreover, the elimination or reduction of the first-order cyclic error contribution to the superheterodyne phase is independent of many aspects of the electronics used to generate the superheterodyne signal. Such aspects include differences in the sensitivity of the detectors used to measure the intensity of the output beams, and differences in the gains of the preamplifiers and/or amplifiers that amplify those intensities and related downstream signals. In addition, the calculation of the superheterodyne signal increases the phase resolution of the interferometer system by a factor of 2.
In one embodiment, for example, the invention features a method that includes: i) directing two beams derived from a common source along different paths in an interferometer, wherein the two beams have orthogonal polarizations and frequencies that differ by a heterodyne frequency; ii) producing a first output beam derived from a portion of each of the two beams having a first common polarization; iii) producing a second output beam derived from a portion of each of the two beams having a second common polarization substantially orthogonal to the first common polarization; iv) generating first and second signals derived from intensity measurements of the first and second output beams, respectively; v) calculating a superheterodyne signal corresponding to a product of the first and second signals to substantially eliminate at least some first-order cyclic errors present in the first and second signals; and vi) extracting the phase of the superheterodyne signal to provide information related to the different paths in the interferometer. For example, the information related to the different paths can be a change in position of an object in one of the different paths or an angular deviation of an input beam from which the beams in the interferometer are derived. Also, the generation of the first and second signals can include passing each of the measured intensities through a high band pass filter.
More generally, in one aspect, the invention features an interferometry method including: directing two beams derived from a common source along different paths; producing a first output beam derived from a first portion of each of the two beams; producing a second output beam derived from a second portion of each of the two beams; and calculating a product of a first signal derived from the first output beam and a second signal derived from the second output beam.
Embodiments of the method may include any of the following features.
The two beams may be directed along the different paths within a distance measuring interferometer, for example, a single-pass distance measuring interferometer or a double-pass distance measuring interferometer. Alternatively, the two beams may be directed along the different paths within an angle measuring interferometer. For example, the angle-measuring interferometer may include a beam shearing assembly.
Calculating the product of the first and second signals may substantially eliminate at least some first-order cyclic errors present in the first and second signals from the calculated product.
The method may further include extracting from the calculated product information related to the different paths. For example, the information may correspond to a change in position of an object in one of the different paths. Alternatively, the two beams may be derived from an input beam, and the information may correspond to an angular deviation of the input beam.
The first portions may have a first common polarization, and the second portions may have a second common polarization different from the first common polarization. For example, the first and second common polarizations may be substantially orthogonal.
Moreover, producing the first and second output beams may include combining the two beams and directing the combined beams to a polarizing beam-splitter to produce the first and second output beams. In other embodiments, producing the first and second output beams may include combining the two beams, directing the combined beams to a non-polarizing beam-splitter to produce first and second intermediate beams, and directing each of the intermediate beams to a polarizer to produce the first and second output beams. In yet further embodiments, producing the first and second output beams may include separating each of the two beams into the portion having the first common polarization and the portion having the second common polarization, combining the portions having the first common polarization to produce the first output beam, and combining the portions having the second common polarization to produce the second output beam.
Also, the method may further include passing one of the directed beams and not the other of the directed beams through a first polarizer prior to producing the first and second output beams. For example, the first common polarization may be a linear polarization, and the first polarizer may be oriented at 45 degrees to the first common linear polarization. The method may further include passing the other of the directed beams through a second polarizer prior to producing the first and second output beams. For example, the first and second polarizers may be oriented to pass orthogonal linear polarizations.
The method may also include directing an input beam derived from the common source to a polarizing beam-splitter to produce the two beams directed along the different paths.
The two beams directed along the different paths may have frequencies that differ by a heterodyne frequency. In such embodiments, the product of the first signal and the second signal may include a superheterodyne term. Furthermore, the method may also include extracting a phase of the superheterodyne signal.
The method may also include generating the first signal by measuring an intensity of the first output beam and generating the second signal by measuring an intensity of the second output beam. Furthermore, generating the first and second signals may also include passing each of the measured intensities through a high band pass filter.
In another aspect, the invention features a lithography method for use in fabricating integrated circuits on a wafer. The lithography method includes: supporting the wafer on a moveable stage; imaging spatially patterned radiation onto the wafer; adjusting the position of the stage; and measuring the position of the stage using the interferometry method described above.
In another aspect, the invention features a lithography method for use in the fabrication of integrated circuits. The lithography method includes: directing input radiation through a mask to produce spatially patterned radiation; positioning the mask relative to the input radiation; measuring the position of the mask relative to the input radiation using the interferometry method described above; and imaging the spatially patterned radiation onto a wafer.
In another aspect, the invention features a lithography method for fabricating integrated circuits on a wafer. The lithography method includes: positioning a first component of a lithography system relative to a second component of a lithography system to expose the wafer to spatially patterned radiation; and measuring the position of the first component relative to the second component using the interferometry method described above.
In another aspect, the invention features a method for fabricating integrated circuits, the method including any of the lithography methods described above.
In another aspect, the invention features a beam writing method for use in fabricating a lithography mask, the method including: directing a write beam to a substrate to pattern the substrate; positioning the substrate relative to the write beam; and measuring the position of the substrate relative to the write beam using the interferometry method described above.
In general, in another aspect, the invention features an interferometry system including: i) an interferometer configured to direct two beams derived from a common source along different paths and produce a first output beam derived from a first portion of each of the two beams and a second output beam derived from a second portion of each of the two beams; ii) first and second detectors positioned to measure an intensity of the first and second output beams, respectively; and iii) an electronic processor coupled to the first and second detectors, wherein during operation the electronic processor calculates a product of a first signal derived from the intensity of the first output beam and a second signal derived from the intensity of the second output beam.
Embodiments of the interferometry system may include any of the following features.
The interferometer may include a distance measuring interferometer, for example, a single-pass distance measuring interferometer or a double-pass distance measuring interferometer. Alternatively, the interferometer may include an angle measuring interferometer. For example, the angle measuring interferometer may include a beam shearing assembly.
The product calculated by the electronic processor may substantially eliminates at least some first-order cyclic errors present in the first and second signals.
During operation the electronic processor may also extract from the calculated product information related to the different paths in the interferometer. For example, the information may correspond to a change in position of an object in one of the different paths. In other embodiments, for example, the interferometer may be configured to derive the two beams from an input beam, and wherein the information extracted by the electronic processor corresponds to an angular deviation of the input beam.
The interferometer may be configured to cause the first portions to have a first common polarization and the second portions to have a second common polarization different from the first common polarization. For example, the first and second common polarizations may be substantially orthogonal.
In some embodiments, the interferometer includes a polarizing beam splitter, and the interferometer is configured to combine the two beams after directing them along the different paths and then direct the combined beams to the polarizing beam-splitter to produce the first and second output beams. In other embodiments, the interferometer includes a non-polarizing beam splitter and two polarizers, and the interferometer is configured to combine the two beams after directing them along the different paths, direct the combined beams to the non-polarizing beam-splitter to produce first and second intermediate beams, and direct each of the intermediate beams to a corresponding one of the polarizers to produce the first and second output beams. In yet further embodiments, the interferometer is configured to separate each of the two beams into the portion having the first common polarization and the portion having the second common polarization, combine the portions having the first common polarization to produce the first output beam, and combine the portions having the second common polarization to produce the second output beam.
The interferometer may also includes a first polarizer, and the interferometer may be configured to further pass one of the directed beams and not the other of the directed beams through the first polarizer prior to producing the first and second output beams. For example, the first common polarization may be a linear polarization, and the first polarizer may be oriented at 45 degrees to the first common linear polarization. The interferometer may also include a second polarizer, and the interferometer may be configured to pass the other of the directed beams through the second polarizer prior to producing the first and second output beams. For example, the first and second polarizers may be oriented to pass orthogonal linear polarizations.
The interferometer may include a polarizing beam-splitter positioned to receive an input beam and produce the two beams to be directed along the different paths.
The interferometry system may include the common source. For example, the common source may be configured to introduce a heterodyne frequency difference between the two beams directed along the different paths by the interferometer. Furthermore, the product calculated by the electronic processor may include a superheterodyne term. Moreover, during operation the electronic processor may extract a phase of the superheterodyne signal. Also, the electronic processor may include a high band pass filter, and during operation the electronic processor may pass the measured intensity for each of the first and second output beams through the high band pass filter to generate the first and second signals.
In another aspect, the invention features a lithography system for use in fabricating integrated circuits on a wafer. The lithography system includes: a stage for supporting the wafer; an illumination system for imaging spatially patterned radiation onto the wafer; a positioning system for adjusting the position of the stage relative to the imaged radiation; and the interferometry system described above for monitoring the position of the wafer relative to the imaged radiation.
In another aspect, the invention features a lithography system for use in fabricating integrated circuits on a wafer. The lithography system includes: a stage for supporting the wafer; and an illumination system including a radiation source, a mask, a positioning system, a lens assembly, and the interferometry system described above; wherein during operation the source directs radiation through the mask to produce spatially patterned radiation, the positioning system adjusts the position of the mask relative to the radiation from the source, the lens assembly images the spatially patterned radiation onto the wafer, and the interferometry system monitors the position of the mask relative to the radiation from the source.
In another aspect, the invention features a beam writing system for use in fabricating a lithography mask, the system including: a source providing a write beam to pattern a substrate; a stage supporting the substrate; a beam directing assembly for delivering the write beam to the substrate; a positioning system for positioning the stage and beam directing assembly relative one another; and the interferometry system described above for monitoring the position of the stage relative to the beam directing assembly.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.